When a vehicle's fuel tank is filled with liquid fuel at the gas station by means of the typical fuel pump nozzle, fuel vapor above the liquid fuel in the interior of the fuel tank is displaced as the fuel level increases. Obviously, this vapor must be vented for the tank to be filled. Because it is undesirably to vent fuel vapor directly into the atmosphere, modern vehicles all employ a vapor recovery system that in its most simplistic form consists of a vent hole in the tank connected via a tube to a vapor recovery canister.
As early vapor recovery systems came into use, it was discovered that a valve of some kind had to be incorporated between the vent hole and the vapor recovery system. Such a valve is necessary because vapor recovery canisters are inundated by liquid fuel that splashes out of a full fuel tank, and, more significantly, liquid fuel would drain out of the fuel tank creating a significant fire hazard in the case of a vehicle rollover.
A variety of fuel vent valves have been developed to address this problem. Such valves are described in U.S. Pat. Nos. 4,966,299; 6,591,857; and 6,634,341. These vent valves employ all manner of floats, springs, and combinations thereof that work to partially or completely close the fuel vent to prevent splashing fuel from entering the vapor recovery system and to completely seal the fuel vent in case of vehicle rollover.
A mechanical floats, springs, and combinations suffer from similar problems. They are mechanical devices involving moving parts, and they have a failure rate depending on assembly failure, material failure, temperature, humidity, wear, etc. of these parts.